Two-way signaling system



Ma1y 5, 1942. B. G. BJRNsoN Two-WAY SIGNALINGsYsTEM Filed May 21, 194;

/A/l/NTQR B. G. BJOR/VSON ATTORNEY Patented May 5, 1942 UNITED STATESyPATENT OFFICE Bell Telephone Laboratories,

Incorporated,

New York, N. Y., a corporation of New York Application May 21, 1941,Serial No. 394,421

Claims.

The invention relates to two-way signal transmission systems andparticularly to circuits for directionally controlling signaltransmission in such systems.

The invention is specifically directed to twoway telephone systemsemploying voice-con.- trolled switching circuits at the terminalstations to give exclusive directional control of transmission to thesubscriber at one terminal who rst starts to speak while at the sametime minimizing signal transmission lock-out and clipping andfacilitating break-in by the subscriber at the other terminal station.In one system of this type disclosed in the copending application of H.J. Fisher, Serial No. 338,884, led June 5, 1940, which issued as PatentNo. 2,251,276 on August 5, 1941, the control circuits at the terminalslimit the transmission frequency range in both directions to only aportion, say one-half, of the signal frequency band in the idlecondition of the system and when the two subscribers associated f withthe two terminal stations both start talking simultaneously or within agiven time interval; and when one subscriber starts talking a denitetime greater than that given time interval in advance of the other givethe former complete control with full-frequency band transmission of hissignal currents while allowing partial-frequency band transmission ofthe speech signals of the latter to the distant terminal to facilitatebreak-in during the pause by the former. This is accomplished by the useof mutually exclusive lters which are effectively switched in and out ofthe incoming and outgoing signal paths at the terminal stations byvoice-operated switching circuits to properly vary the transmissionfrequency ranges over the system in both directions.

An object of the invention is to improve systems of the above-describedgeneral type so as to produce equivalent results from the standpoint ofminimizing transmission lockout and facilitating break-in with economyof apparatus and improved quality of signal transmission.

The Various objects and features of the invention will be betterunderstood from the following detailed description when read inconjunction with the accompanying drawing, the single iigure of whichshows one modification of the invention embodied in a two-way radiotelephone system.

The two-way radio telephone system of the drawing comprises a westterminal station and an east terminal station connected by a radio linkwhich preferably employs dilferent radio frequencies for the oppositedirections of transmission to reduce the possibility of singingdifficulties. Each station includes a four-wire circuit connecting atwo-way telephone line to a radio transmitter and a radio receiver.

The four-wire circuit at the west station comprises, as shown, a,transmitting circuit TCW including an automatic volume control deviceTVW, which may be a variable-gain amplier of the type known as a Vogad(volume-operated gainadjusting device), for maintaining the transmittedsignal volume substantially constant, and the delay circuit DCW leadingto the radio transmitter RTW with an associated amplifier, and areceiving circuit RCW including the receiving loss control device RLWleading from the radio receiver RRW. The input of the circuit TCW andthe output of the circuit RCW are coupled by the usual hybrid coil HWand associated balancing network Nw to a two-way line section which maybe connected directly or through a switchboard (not shown) to a two-waytelephone circuit TLW leading to a west subscribers telephone stationindicated by a box.

'Ihe four-wire circuit at the east station comprises similar circuitsand apparatus as described above for the west station as indicated bythe use of similar identification characters for the correspondingelements with the subscript E instead of W.

At the west station, the receiving circuit RCW includes at some pointbetween 'the radio receiver RRW and the receiving loss control deviceRLw, two parallel, conjugate circuit branches coupled by the hybrid coilH1 and associated balancing network N1 andthe hybrid coil Hz andassociated balancing network N2, respectively, to the input and outputportions of the circuit RCW. One of these circuit branches designated l,which includes the lter 2 and the one-way amplifying device 3, isnormally disabled in its input by a short-circuiting connection throughthe normally closed contacts of the unenergized switching relay 4,whereas the other branch 5 is normally operative due to the normallyclosed condition of the contacts of unenergized switching relay 6, inseries with its conductors. The transmitting circuit TCW is normallydisabled at a point between delay circuit DCW and the radio transmitterRTW by a short-circuiting connection across its conductors through thenormally closed switching contacts of the unenergized switching relay 1.

Bridged across the transmitting circuit TCW of the west station, at apoint between vogad having its operating winding connected across.

the output of the latter, the normally closed contacts of the latterrelay being connected in series in the conductors of the, transmittingswitching circuit 8 at a point inthe output of amplifier-detector It.

Similarly, at the east station the receiving circuit RCE includes atsome points between the radio receiver RRE and the receiving losscontrol device RLE, two parallel,y conjugate circuit branches coupled bythe hybrid coil H3 and associated balancing network N3 and hybrid coilH4 and associated balancing networkNi, respectively, to the input andoutput portions of the receiving circuit RCE, One ofthese parallelcircuit branches designated as Hl, which includes the lter i5 and theone-way amplifying device ES, is normally disabled in its input by ashortcircuiting connection through the normally closed contacts ofunenergized switching relay i?, and the other circuit branch designatedi8 is normally operative due tO the normally' closed condition of thecontacts of unenergized switching relay it, connected in series with itsconductors. rEhe transmitting circuit TCE is normally disabled at apoint between the delay circuit DCE and the radio transmitter RTE by ashort-circuiting connection through the normally closed contacts ofunenergized switching relay 2D.

Bridged across the transmitting circuit TCE of the east station at apoint between vogad TVE and delay circuit DCE is the input of aVoice-operated transmitting switching circuit 2l including in order, thefilter 22, the amplier-detector Z3 and the operating windings ofswitching relays il', I9 and 2i! connected in parallel to the output ofthe latter. Bridged across theoutput of the circuit branch I8 of thereceiving circuit RCE, beyond the point of connection of the contacts ofswitching-relay I9, is the, input of a voice-operated receivingswitching circuit 26! including the ampliiier-detector and the operatingwinding of the switching relay 26 connected to the output of the latter,the normally closed switching contacts of the latter relay beingconnected in series with the conductors of the transmitting switchingcircuit 2I at a point in the output of amplifier-detector 23.

The iilter 2 in branch I 0f the receiving circuit RCW at the weststation and the filter I5 in the branch I4 of the receiving circuit RCEat the east station are mutually exclusive, each passing a differentportion, which is preferably approximately one-half, of the voice signalfrequency band. The filter S in front of the amplifier-detector lil inthe transmitting switching circuit 8 at the west station, passesthe samefrequencies as the filter I5 in the receiving circuit RCEat the eaststation, and the filter 22 in front 0f the amplier-detector 23 in thetransmitting switching circuit 2i at the Veast station passes.` the samefrequencies as the iil'ter 2 in the receiving circuit RCW at the weststation. rIhus, the two lters used at each station', filters 2 and 9 atthe West station and lters I5 and 22 at the east station, are alsomutually exclusive.

In tests made to determine the best splitting up of the voice frequencyband, 250 to 2860 cycles per second, by the filters in the signaltransmission paths at the two stations from the standpoint of equalnaturalness and articulation in each direction, the arrangement whichproved to be most satisfactory was one in which the voice frequency bandwas split up into four subbands as follows:

250 to 500 cycles 50 to i000 cycles i000 to 2G90 cycles 200G tc 2830cycles With this arrangement, the lter in the receiving transmissioncircuit at one station and the transmitting switching circuit filter atthe other station would be designed to transmit the rst and thirdsubband specified, and the receiving transmission circuit filter at theother station and the transmitting switching circuit filter at the firststation would be designed to pass the second and fourth subbandsspecied. For example, each of the filters Iii and 9 might be designed totransmit the frequency ranges 250 to 560 cycles and 1000 tc 200i)cycles, and to suppress the other frequencies'in the voice frequencyband, and each of the iilters 2 and 22 might be designed to transmit thefrequency ranges 5Go to 1G00 cycles and 2096 to 28610 Vcycles and tosuppress the other frequencies in the voice frequency band. Forconvenience in descrip-tion, in the drawing the filters It and dtransmitting the other, frequency subbands specified above aredesignated as type No. l iilters and the filters 2 and 22 transmittingthe other frequency subbands specified above are designated as type No.2 filters, it being understood that the type No. l iilterg,l transmitapproximately one-half of the frequencies in the voice frequency bandwhereas rthe type No. 2 filters transmit the other half of thefrequencies in the voice frequency band.

As in the usual vodas circuit., the relays operated by the transmittingswitching branch at each station preferably should have a certain amountof hang-over in their operations to keep them operated during wealrintermediate speech sounds between stronger syllables and during weakendings. This hang-over normally would be about O lZ second for thesinging suppressor relays 7, 2o and about ttl? second for the echosuppressor relays il, li and il, Iii. The receiving switching relays it,26 should also have a certain amount of hang-over, the amount of whichdepends on the length of the connected land line Lw or TLE. According tostandard practice,

this hang-over is kept as shortIa-s possible (usually about (m20 second)to minimize the duration of false operations on received static. Due tothe unequal hang-over a talker may keep control of his transmittingterminal a greater part of the time than he will control thecorresponding receiving terminal. Occasionallyy in practice thesensitivity of the receiving amplifier-detector must be lowered clue tostatic conditions, which further increases the inequality of control ofthe ransmitting and receiving` terminals. Under these conditions, withthe usual vodas switching circuit, assuming west is speaking, it isquite easy forfeast to gain control of his transmitting terminal whilewest continues to control his own transmitting terminal. Thus a lock-outwill result with both west and east controlling only their respectivetransmitting terminals. This lock-out normally would continue until onetalker stops and loses control of his terminal.

In the circuits of the invention as shown in the drawing, such lock-outsare eliminated and break-in facilitated with a relatively small amountof transmission control apparatus and with high quality of signaltransmission in a manner which will be brought out in the followingcomplete description of the operation of the system of the drawing.

In the absence of signal transmission over the system in eitherdirection, with the transmitting and receiving switching circuits at thetwo stations unoperated, the transmitting speech circuit TCW at the weststation and the transmitting speech circuit TCE at the east station areboth blocked, as indicated in the drawing, so that neither can transmitto its radio transmitter. The receiving circuit RCW at the west stationand the receiving circuit RCE at the east station are normallyconditioned to pass the entire voice frequency range due to the normaloperative condition oi the receiving circuit branches and I8,respectively, at the two stations, which do not contain any filters, andthe normally blocked condition of the branches I and i4 containingfilters, at the respective stations.

Now, let it be assumed that speech signals from a west telephonesubscriber are received at the west station over the associated two-waytelephone line TLW and are impressed by the hybrid coil Hw on the inputof the transmitting circuit TCW, and at that time no speech signals arebeing received at the east station over the associated two-way telephoneline TLE from the east subscriber. The west subscribers speech currentsin the circuit TCW will pass through the transmitting vogad TVW in whichthey will be regulated to the desired constant volume. The speechcurrents in the output of the vogad TVW will be divided between thedelay circuit DCW in the speech transmission path and the input of thetype No. l filter 9 in the transmitting switching circuit 8.

Of the speech signal waves diverted into the transmitting switchingcircuit 8, only part, approximately one-half, of the frequencycomponents in the signal frequency band will be transmitted by the typeNo. 1 filter 9. The passed signal frequencies will be amplified anddetected in the amplifier-detector Ill and cause the operation of thetransmitting switching relays 4, 5 and 'I in its output. The operationof relay 4 will remove the normal short across the output of the lterbranch I of the receiving circuit RCW and the simultaneous operation ofrelay B will disable the input of the non-filtering branch 5, thuseffectively limiting the transmission frequency range of the receivingcircuit RCW at the west station to the frequencies passed by the typeNo. 2 filter 2 in the circuit branch I, and preventing transmission ofthe remaining frequencies in the signal frequency band corresponding tothose passed by the type No. 1 filter 9 in the transmitting switchingcircuit 8.

The simultaneous operation of the switching relay 1 will remove thenormal short across the transmitting circuit TCW in the output of delaycircuit DCW, allowing transmission of wests speech currents, whichmeanwhile have been delayed in that delay circuit, out over thetransmitting circuit TCW to the radio transmitter RTW which will radiatethem to the east station.

At the east station, wests speech signals will be picked up and detectedby the radio receiver RRE, and the detected signals will pass throughthe hybrid coil H3 in circuit RCE. If the transmitting switching circuitZI at the east station is in the unoperated condition, indicating thatthe east subscriber has not started to talk after west within a timeinterval equal to the over-all one-way transmission time between thewest and east stations, the iilter branch I4 of the receiving switchingcircuit RCE will be in its normally blocked condition and thenon-ltering branch I3 will be operative, due to the unoperated conditionof the transmitting switching relays I1 and I8, respectively at the eaststation. Thus, the entire frequency range of wests speech currentspassing through the hybrid coil H3 will be transmitted over thereceiving circuit branch I3 through hybrid coil H4 and the receivingloss control device RLE, and then through hybrid coil I-IE to thetwo-Way telephone line TLE over which it will be transmitted to the eastsubscriber.

Meanwhile, a portion of wests speech currents will be diverted from thereceiving circuit branch IB into the receiving switching circuit 24,will be amplied and detected by the amplier-detector 25 therein andcause the operation of the receiving switching relay 26 to disable theenergizing circuit for the transmitting switching relays I'I, I@ and 2Bat the station. West now has obtained exclusive control of the switchingcircuits at both stations of the systems. Thus, any subsequentlyreceived speech currents from the east subscriber entering thetransmitting switching circuit 2i and operating the ampliiier-detector23, therein, cannot operate the transmitting switching relays to takedirectional control of the station away from west as long as westcontinues to talk without an appreciable pause. The blocked condition ofthe transmitting circuit TCE with the transmitting switching relay 20deenergized will prevent any later received speech currents from east,or any echoes of wests speech currents entering the transmitting circuitTCE due to any unbalance of the hybrid coil set HE, NE, from beingradiated by the radio transmitter RTE.

Now, let it be assumed that the east subscriber starts to talk about thesame as the west subscriber or Within a time interval thereafter withinthe over-all one-way transmission time between the West and eaststations.

Easts speech currents received at the east station over the line TLEwill be impressed by the hybrid coil HE on the input of the transmittingcircuit TCE, and after regulation to constant volurne in thetransmitting vogad TVE, will be divided between the delay circuit DCE inthe transmitting circuit proper and the input of the type No. 2 lter 22in the transmitting switching circuit 2|.

That part of the frequency band of easts speech currents passed by thetype No. 2 lter 22 in circuit 2| will be amplied and detected byamplifier-detector 23 and, the receiving switching relay 24 being in itsnormally unoperated condition due to the fact that wests speech currentshave not yet been received at the east station, will operativelyenergize the transmitting switching relays II, I9 and 29. Relay I"I willoperate to rem-ove the normal short across the filtering branch I4 ofthe receiving switching circuit RCE, and the switching relay I9 willsimultaneously operate to disable the non-filtering branch I3 of thatcircuit, so that the frequency transmission range of the receivingcircuit RCE is thus effectively limited to that portion of the `speechfrequency range passed by the typeNo. l

filter I in branch I4.

Transmitting switching relay 2li will simultaneously operate to removethe normal short across the transmitting switching circuit TCE, therebyallowing the complete frequency band of east speech currents Which havebeen meanwhile delayed in delay circuit DCE to be transmitted to theradio transmitter RCE and radiated thereby to the west station.

Wests speech currents arriving at the east station will be picked up anddetected in the radio receiver RRE and will be transmitted through thehybrid coil H3 of the receiving circuit RCE, Because of the previousoperation of the transmitting switching relays I'I and I9 in response toeasts speech currents to disable branch I8 and enable branch I4, half ofthe frequency band of wests speech currents will be suppressed and theother half passed by the type No. 1 lter I5 in the operative branch E4.The part-band currents in the output of lter I5 in branch |14 will beamplified by the amplifying device I6 in that branch, provided for thepurpose to be described later; and will be transmitted through thehybrid coil H4, the receiving loss control device RLE and hybrid. coilHs to the associated two-way line TLE over which it will be transmittedto and heard by the east subscriber.

Because of the connection of the receiving switching circuit 24 to theoutput of the now disabled branch i3 of the receiving circuit RCE, whichbranch is in conjugate relation with the receiving branch lll, westsspeech currents will not be able to cause false operation of theampliiier-detector 25 in that switching circuit to disable the output ofthe transmitting switching circuit 2l. Thus easts speech waves(full-band) will continue to be transmitted from the east station.

At the west station, easts speech waves (fullband) will be picked up anddetected by the radio receiver RRW, and will be transmitted over thereceiving circuit RCW through the hybrid vcoil H1. Because of theprevious operation of the transmitting switching relays 4 and E at thewest station in response to wests outgoing speech currents in the mannerpreviously described, the

iiltering branch I of the receiving circuit RCW is now operative and thenon-ltering branch 5 is now disabled. Therefore, only that portion ofeasts speech currents passed by hybrid coil Hi, within the transmissionfrequency of the type No. 2 filter 2 in the branch I will betransmitted. Easts speech currents (half-band) in the output of ilter 2will be amplified by the one-way amplier 3 provided in the branch l forthe purpose to be described later, and then will be transmitted throughthe hybrid coil H2 and through the receiving loss control device RLW.The waves in the output of RLW will be transmitted through the hybridcoil Hw to the associated two-way line TDW over which they will betransmitted to the West subscriber and will be heard by him..

Because of the connection of the receiving switching circuit i i to theoutput of the disabled branch 5 of the receiving circuit RCWV and theconjugate connection of the output of the branch 5 with respect to theoutputiof the branch l, it will be apparent that easts half-band speechcurrents cannot get through to falsely operate the receivingamplifier-detector IZand. thus receiving switching relay I3 to reversedirectional control at the west terminal station.

It will be apparent from the above description of operation that wheneither subscriber starts talking before the other subscriber by a timeinterval greater than the over-all one-way transmission time between theterminal stations, the first subscriber to talk gets exclusive controlof the switching circuits at both terminal stations and is able totransmit his voice currents with full-frequency band transmission; thatwhen both subscribers start talking at once or within a time intervalequal to the over-all one-way transmission time between terminalstations, each subscriber will gain control of the transmitting circuitat his'own terminal and will be able to transmit therefrom his completesignal frequency band, but will only partially block the receivingtransmission circuit at his own terminal, so that the speech currents ofeach can get through and be heard by the other subscriber with part-bandtransmission.

The use of a lter in the transmitting switching circuit at each terminalwhich passes a different frequency range than the iilter in thefiltering branch of the receiving circuit at that terminal, serves toprevent false operation of the transmitting switching circuit at eachterminal by echoes of incoming speech at the termination of periods ofparteband transmission. The purpose of the amplifier 3 in the lteringbranch I of the receiving circuit RCW at the west station and of theampliier I6 in the nltering branch M of the receiving circuit RCE at theeast station is to provide extra gain in the passed band during partbandtransmission so as to provide about equal lcudness of the receivedsignal for whole or part-band transmission. This was found to providebetter reception at the terminal and more naturalness in theconversation.

The system of the invention as described above is an improvement overthe system disclosed in the aforementioned Fisher patent application inthat it provides the same protection against transmission loclr-out andsubstantially the same facility of break-in as in the latter system withthe elimination of two iilters at each terminal, which results, inaddition to the economy of apparatus, in a substantial improvement inquality of transmission due to avoidance of transmission impairment dueto changes in frequency characteristics and delay distortion in thefilters eliminated, and also provides better reception than in thelatter system during parteband transmission due to the additional gainpresent in the receiving circuit for that condition of operation.

Although in the above description of the system of the invention it isspecified that each of the mutually exclusive filters in thetransmission paths and switching circuit be designed to transmitapproximately half of the voice frequency band and particular frequencyranges are set forth, it is to be understood that these iilters may bedesigned to transmit other different portions of the voice frequencyband and other frequency ranges greater or less than the amountsspecified. Various other modifications of the circuit illustrated anddescribed Which are within the spirit and the scope of the inventionwill appear to persons skilled in the art.

What is claimed is:

l. In a two-way telephone system including at each terminal stationoppositely directed oneeway transmission paths for the telephone signalsrtransmitted in opposite directions, directional control meanscomprising at each station means normally disabling the transmittingpath while allowing the receiving path to transmit substantially allfrequencies in the voice frequency range, a transmitting switchingcircuit responsive to outgoing signals in said transmitting path, in theabsence 'of prior incoming signals in the receiving path, to render saidtransmitting path operative to transmit substantially all frequencies inthe voice frequency range, and to reduce the transmission frequencyrange of said receiving path to only a part, different for each terminalstation, of the voice frequency range, and a receiving switching circuitresponsive to incoming signals in said receiving circuit only when saidtransmitting switching circuit is unoperated, to disable saidtransmitting switching circuit.

2. The system of claim 1, in which the transmitting switching circuit ateach station is responsive only to frequencies in the voice frequencyrange outside the reduced frequency range of the receiving transmissionpath at the station when the transmitting switching circuit thereat isoperated.

3. A control terminal for a radio telephone system comprising a voicefrequency transmitting path leading to a radio transmitter and a voicefrequency receiving path leading from a radio receiver, saidtransmitting path being normally disabled from transmitting to the radio4 transmitter, said receiving path being normally operative to transmitfromv said radio receiver to a telephone receiver substantially allfrequencies in the voice frequency band, a transmitting switchingcircuit responsive to outgoing telephone signals in said transmittingpath, in the absence of prior receiver telephone signals in saidreceiving path, to render the transmitting path operative to transmitsubstantially all frequencies in said outgoing signals, and to reducethe transmission frequency range of said receiving path to a portion ofthe voice frequency band, a receiving switching circuit responsive toincoming telephone signals in said receiving path, only when saidtransmitting switching circuit is unoperated, to disable saidtransmitting switching circuit, and means in said transmitting switchingcircuit for preventing its operation by frequencies in the voicefrequency band, corresponding to those in the reduced transmissionfrequency range of said receiving path when said transmitting switchingcircuit is operated.

4. The control terminal of claim 3, in which said transmitting switchingcircuit is responsive to said outgoing telephone signals in saidtransmitting path to remove the normal disability therefrom and toeffectively insert in said receiving path a filter passing only a partof the voice frequency band, and the last-mentioned means comprising afilter in the input of said transmitting switching circuit adapted topass only frequencies in the voice frequency range suppressed by thefirst filter.

5. A two-way signal transmission system comprising two oppositelydirected one-way transmission paths for transmitting signals in oppositedirections between the terminals of the system, each of said pathsexcept for a normal disability near the input thereof being operative totransmit substantially the entire signal frequency band, transmittingswitching means connected to each path in front of the disabling pointtherein, responsive to signals impressed on the input of that path, inthe absence of prior incoming signals in the output of the other path atthe same terminal, to cause the normal disability in the input of thefirst path to be removed, and the transmission frequency range of theoutput portion of the other path at the same terminal to be limited toonly a portion of the signal frequency band, different for eachterminal, receiving switching means responsive to incoming signals inthe incoming path at each terminal only when the transmitting switchingmeans at the same terminal is unoperated by prior outgoing signalsthereat, to disable the transmitting switching means at that terminal,and means preventing operation of the transmitting switching means ateach terminal by waves of frequencies corresponding to the signalfrequencies Within the limited frequency range of the incoming path atthat terminal when the transmitting switching means thereat isoperated.`

6. A two-way telephone transmission system comprising at each terminaloppositely directed one-way transmission paths for the telephone signalstransmitted in opposite directionspthe transmitting path being normallydisabled and the receiving path being normally operative to transmitsubstantially all frequencies in the telephone frequency range, atransmitting switching circuit responsive to outgoing telephone signalsin the transmitting path, in the absence of prior incoming telephonesignals in the receiving path, to remove the normal disability from thetransmitting path so that it is operative to transmit substantially thewhole frequency range of said outgoing signals, and to decrease thetransmission frequency range of the receiving path to only a portion,different for each terminal, of the telephone signal frequency range, areceiving switching circuit responsive to incoming telephone signals inthe receiving path in the absence of prior outgoing telephone signals inthe transmitting path to disable the transmitting switching circuit, andmeans for preventing operation of the transmitting switching circuit bywaves of frequencies corresponding to those transmitted by the receivingpath when its transmission frequency range is reduced.

7. The system of claim 6, in which the receiving path at each terminalcomprises two parallel conjugately connected branches one normallyoperative to transmit all frequencies in the signal frequency range andthe other normally disabled and including a filter adapted to transmitonly said reduced portion of the telephone frequency range allocated tothat terminal, said transmitting switching circuit operating to reducethe transmission frequency range of said receiving path by disablingsaid one branch and removing the disability from the other branch, thereceiving switching circuit at both terminals being controlled from apoint in said one branch beyond the disabling point therein so as onlyto be operated by incoming signals when said one branch is operative,and the last-mentioned means comprising another filter in the input ofthe transmitting switching circuit adapted to transmit only thosefrequencies in the telephone signal frequency range suppressed by theother filter at the terminal.

8. The system of claim 6 in which the receiving path at each terminalcomprises two parallel conjugately connected branches one normallyoperative to transmit substantially all frequencies in the telephonesignal frequency range and the other normally disabled and including anlter adapted to transmit only said decreased portion of the telephonefrequency range, said transmitting switching circuit at each terminaloperating to decrease the transmission frequency range of the receivingpath at that terminal by disabling said one branch and enabling saidother branch thereof, and said other of said conjugately connectedbranches of the receiving path at each terminal includes in the outputof the filter therein amplifying means such as to give the telephonesignals of decreased frequency range transmitted by that branch a degreeof loudness substantially equal to that of the telephone Waves ofgreater frequency range transmitted by said one branch at the terminalwhen it is operative.

9. A two-way telephone system comprising two oppcsitely directed one-waypaths for respectively transmitting the voice signals of telephonesubscribers at the two terminal stations to the telephone subscriber atthe other station, each path except for a normal disability near itsinput being operative to transmit the entire voice frequency range, afilter at each terminal station respectively adapted to transmit adifferent limited portion of the voice frequency range, a 25transmitting switching circuit connected to the outgoing path at eachstation in front of the disabling point therein, responsive to theoutgoing voice signals of the subscriber at `that station, in theabsence of prior incoming signals in the incoming path at the station,to remove the normal disability from said outgoing path so as to allowtransmission of the complete frequency range of the outgoing signals tothe other station, and to effectively insert the iilter at the rststation into the incoming path thereat to limit its frequencytransmission range, and a receiving switching circuit at each stationresponsive to incoming telephone signals in the incoming path thereat,only when the latter path is operative to transmit the entire voicefrequency range, tov disable the transmitting switching circuit at thesame station.

l0. The system of claim 9, in which the transmitting switching circuitat each station includes a second lter making it selectively operativeonly to the applied frequencies in the Voice frequency range outside thepass frequency range of the other lter at the station.

BJORN G. BJORNSON.

